Megapixels are the digital camera market’s equivalent of horsepower and megahertz—a single metric that consumers and marketers latch on to tenaciously, despite the fact that it hardly describes overall performance. Over the last several years, camera manufacturers have been pumping up the megapixels on each successive camera model, regardless of whether such increases offered any real benefits (hint: they usually did not).
See, throwing more megapixels at the digital imaging problem is akin to bumping up the processor speed on a motherboard with a slow bus and small amounts of RAM, or adding a turbo to a small engine on a car with lousy brakes and wobbly suspension. The number of megapixels in a camera’s image sensor is just one in a number of aspects that truly define how well a camera works.
In an image sensor, larger pixels mean better light-gathering capability. This translates to better low-light performance, better color accuracy, and in some cases better dynamic range. Sensors commonly come in a few different sizes: Full frame (24 x 36mm), APS-C (17 x 25mm), Four-thirds (13.5 x 18mm), and even smaller sensors on compact point-and-shoot models. As manufacturers cram more pixels on a given sensor, those pixels get smaller and smaller.
Based on other things I’ve read that seems right. I’m still using a 6 MP camera with wonderfully low pixel density. My main interest in a newer camera like the Nikon D90 would be for better high ISO performance and more sophisticated autofocus, not the megapixels.